Technical Field
The present disclosure relates to a method and an apparatus for transmitting data to a plurality of beams through a plurality of transmit feeds, and to mitigating inter-beam interference. The disclosure further relates to a method and an apparatus for receiving data transmitted via wireless transmission to a plurality of beams through a plurality of transmit feeds, and to synchronization at the terminal location. The disclosure is particularly though not exclusively applicable to multi-beam broadband communication systems, in particular providing interactivity between the terminal side and the transmitter side, and in particular to high throughput, multiple beam satellite communication systems, e.g., for providing internet services.
Description of the Related Art
Telecom satellite systems providing multiple spot-beam (or simply beam) coverage can substantially increase the system capacity by re-using the available frequency spectrum among the beams. If different signals are transmitted to a multiplicity of beams in order to provide point-to-point interactive services, increasing the frequency re-use leads to a large increase in intra-system interference between the beams (inter-beam interference), which renders the use of the additional spectrum futile. Intra-system interference is generated by the sidelobes of the co-channel beam radiation patterns.
To address the issue of high inter-beam interference in an aggressive frequency re-use multi-beam configuration, joint processing of the signals intended for transmission to the different beams can be carried out at the forward link transmitter (usually the gateway (GW) or hub). This processing, referred to in the following under the generic term “precoding”, intends to ‘revert’ the impact of the satellite channel and interferences. This way the additional spectrum can be exploited and a much higher system capacity can be delivered. A precondition for precoding to work is that the forward link receivers (satellite terminals, also referred to as user terminals (UTs) or simply terminals) provide accurate and timely reports of their channel (channel state information represented by a channel state vector, or simply channel vector) back to the transmitter, which the transmitter uses to form the appropriate precoding matrix.
Various flavors of precoding are known in the prior art and are adopted, e.g., in terrestrial cellular radio standards such as the LTE (Long Term Evolution) and LTE-Advanced and fall under the broad term of multi-user multiple-input multiple-output (MU-MIMO) techniques. C. Lim et al., “Recent trend of multiuser MIMO in LTE-Advanced,” IEEE Commun. Mag., pp. 127-135, March 2013 discloses an example of such a precoding technique. In contrast to MU-MIMO techniques in LTE-Advanced, the scale of the problem dealt with in high throughput multi-beam satellite communication systems is much larger since precoding in the satellite context involves tens or hundreds of satellite antenna feeds (transmit feeds) with corresponding spot beams formed on ground, each serving at least one fixed UT in each epoch.
As regards precoding, the prior art has been to a large extent preoccupied with evaluating various linear and non-linear precoding techniques over the multi-beam satellite channel in order to assess which one approaches the optimum dirty paper coding (DPC) bound described in M. Costa, “Writing on dirty paper,” IEEE Trans. Inf. Theory, vol. 29, no. 3, pp. 439-441, May 1983. In terms of choice of precoding techniques, it has been found in D. Christopoulos, S. Chatzinotas, G. Zheng, J. Grotz and B. Ottersten, “Linear and nonlinear techniques for multibeam joint processing in satellite communications,” EURASIP Journal on Wireless Communications and Networking, 2012 that simple linear techniques already grasp the largest part of the potential multi-user gains with manageable complexity and deliver improvements that at least double the throughput of existing systems. Joint processing at the gateway in the form of precoding is also possible whenever the system uses multiple beams to transmit common information in a point-to-multipoint fashion, as in the case for example of broadcasting or multicasting services. For such systems, precoding aims at elevating the worst-case signal-to-noise receiver within each beam and can be applied provided there is feedback from the receivers.
However, a new problem arises when not considering terrestrial applications, but broadband satellite communication systems. In these, allowing for high throughput while at the same time enabling interactivity between the plural UTs and the GW requires that each data stream transmitted from an antenna feed (transmit feed) towards a spot beam on ground is addressed to multiple UTs and is acting as a container of their data to provide a high degree of statistical multiplexing within the physical layer frame. In such a framework conventional precoding algorithms addressing a single UT per beam are no longer feasible and precoding algorithms addressing multiple satellite terminals with a single precoding matrix are required. Possible frameworks of this kind include the multiplexing of multiple users' data within the DVB-S2 (digital video broadcasting-satellite second generation) base band frame in order to achieve a high framing efficiency, as well as the base band frame of the BGAN (Broadband Global Area Network) standard for mobile satellite services. What is thus needed is a precoding scheme for a communication system in which each of a plurality of data streams transmitted from an antenna feed towards a respective spot beam on ground is addressed to multiple UTs.
On the UT side, employing such a precoding scheme necessitates a dedicated scheme for synchronization, in order for the UT to be able to determine its channel (channel state vector), which needs to be provided to the GW to enable precoding. This is particularly challenging for system configurations in which a background of strong interference by the signals of other beams is present because of an aggressive frequency re-use scheme.